The Motion and Distribution of the Sun-spots 11 



The process einploj'^ed by the Maunders is very convenient when only aver- 

 ages of velocities are to be computed. For reasons that will be discussed later on, 

 I have preferred to compute a daily drift of each spot from the co-ordinates on two 

 consecutive daps. 



The Maunders have also determined the solar rotation-period from recur- 

 rent spots. 



From the investigation now discussed, it appears with all desirable clearness 

 that »the differences of rotation-period of the various groups in any particular 

 zone of latitude are much greater than the differences of mean rotation between 

 the different zones». This fact has, it is true, been observed before, but from 

 the correlation table between synodic rotation-period and latitude given by the 

 Maunders it has been clearly proved. From this we see that the spots are sub- 

 ject to considerable proper motions in longitude as well as in latitude, which fact 

 has first been shown b_y Carrington. Whether the distribution of the motions is 

 subject to any law, or whether it only follows the usual Gaussian law of errors, does 

 not appear from the paper of the Maunders. Carrington, it is true, declares 

 that the latitude motions of the spots are distributed in such a way that spots 

 situated moie than 20 degrees from the equator, are upon the whole, moving 

 towards the poles, and that the motions betweea 10 and 20 degrees are, in general, 

 directed towards the equator. The mean of the motions towards the poles is about 

 2' a day, whereas the motions towards the equator only amount to hàlf. As- far as 

 I know, this statement has never since been verified. 



A couple of the more interesting results which the Maunders have obtained 

 in their paper, are the following : 



»The recurrent spots give a somewhat longer period in the mean, and are more 

 accordant inter se than are the groups treated separately. » 



»The curve given by the different rotation-periods, is not precisely symmetrical 

 with respect to the equator, the zone of shortest rotation being north of the equator* 



For the daily rate of rotation the Maunders give the expression 

 a = 866'.6 — 128 sin'^'ß, 

 computed from recurrent spots. 



Using the material the Maunders have brought together in their correlation- 

 table between rotation-period and heliographic latitude, Hirayama ^ has obtained 

 some interesting results, which in a later chapter will be further discussed. He 

 finishes his work with the following words : 



»The present investigation, though cursory, leads me to conclude that there 

 are two apparent drifts in the motions of the sun-spots. The angular velocity of 

 drift I is represented by 



^ = 140.37 — 2".97 sin^ß, 



' S. Hirayama: The systematic motions of sun-spots. Journal of the College of Science, 

 Imperial University of Tokyo Vol. XXXII, 1912. 



